7.6 Due to incomplete coverage and the non-compulsory nature of available administrative (indirect) data sources, post-censal quarterly estimates of interstate migration have long been considered the weakest measure of a component of population change. For this reason, the model for generating post-censal estimates of interstate migration is largely superseded when new Census information becomes available (i.e. rebased to the Census).

7.7 Part of the process of rebasing Census counts for the estimated resident population (ERP) of the states and territories is the re-derivation of interstate migration for the intercensal period. The overall approach is to minimise state intercensal discrepancy using information from the two Census questions on usual residence one-year ago and five year ago to estimate interstate movements. Where this Census information does not reduce the intercensal discrepancy, the rebased interstate migration estimates remain largely unchanged from the Medicare-based model.

7.8 For example, during rebasing of interstate migration estimates to the 2006 Census as seen in Table 7.1, for New South Wales, Victoria and Queensland the Medicare-based estimate was used in the re-derivation as it reduced intercensal discrepancy. Whereas for South Australia, Western Australia and the Australian Capital Territory the Census-based estimate was used in the re-derivation. For Tasmania and the Northern Territory a combination of Medicare and Census-based estimates were used to reduce the intercensal discrepancy. In these cases where the intercensal error was negative and closer to zero than the difference then the intercensal error was subtracted from the Medicare-based estimate and used for the re-derivation. For more information on the revision of interstate migration estimates, see paragraphs 2.66 to 2.76.

7.9 Interstate flows however, must always sum to zero, as for each arrival there should always be a corresponding departure. Therefore, an additional change is made to scale interstate migration levels for each state and territory so the total will sum to zero at the Australia level as seen in Table 7.1. This adjustment also takes into account all movements for each state and territory before prorating.

7.10 When the intercensal discrepancy is finalised the difference between the original interstate migration estimates and the rebased estimates is apportioned across all quarters by single year of age, sex and movement direction (arrivals/departures) in the intercensal period in order to minimise quarterly change.

7.1 Rebasing interstate migration estimates - 2001-06

Compare Census and Medicare data

Process for intercensal discrepancy

Final intercensal error

Census-based migration estimate(a)

Medicare-based migration estimate(b)

Difference(c)

Re-derivation

Zero-sum adjustment

Final intercensal discrepancy

New South Wales

2 767

-125 465

-136 330

10 865

-136 330

-140 564

-1 467

Victoria

-36 229

-13 121

-2 197

-10 924

-2 197

-5 086

-39 118

Queensland

-24 034

147 890

164 362

-16 472

164 362

160 495

-27 901

South Australia

-12 780

-10 192

-12 639

2 447

-10 192

-11 346

-11 487

Western Australia

-7 259

3 993

-1 399

5 392

3 993

2 715

-3 145

Tasmania

-793

4 113

3 105

1 008

3 898

3 329

-569

Northern Territory

-2 948

-4 514

-8 474

3 960

-5 526

-6 196

-670

Australian Capital Territory

-4 673

-2 375

-6 428

4 053

-2 375

-3 169

-1 414

Other Territories

234

-329

. .

-329

-165

-178

56

Australia

-85 715

0

0

0

15 468

0

-85 715

Sum(d)

. .

. .

. .

. .

. .

. .

85 771

. . not applicable

(a) Census estimates have been adjusted for net undercount.

(b) Medicare-based estimates accumulated quarterly.

(c) Difference between Census-based and Medicare-based estimates.

(d) Sum of absolute values for states and territories excluding Other Territories.